Retention ponds are basins that maintain a low level of water consistently then fill with storm water when it rains. After the storm has passed, the retention ponds release the water into the surrounding environment at a slow pace over the course of a few days until the ponds return to their original water level. Retention ponds reduce the strain of heavy rainfall by allowing the water to collect and dispense gradually rather than flood all at once, causing downstream flooding and the erosion of stream banks. Retention ponds also promote the occurrence of natural processes, such as the settling of particulates due to gravity and the decomposition or uptake of pollutants by algae and plants.